The continuous trend in the development of electronic devices has been to minimize the sizes of the devices. While the current generation of commercial microelectronics are based on sub-micron design rules, significant research and development efforts are directed towards exploring devices on the nano-scale, with the dimensions of the devices often measured in nanometers or tens of nanometers. Besides the significant reduction of individual device size and much higher packing density compared to microscale devices, nanoscale devices may also provide new functionalities due to physical phenomena on the nanoscale that are not observed on the microscale.
For instance, electronic switching in nanoscale devices using titanium oxide as the switching material has recently been reported. The resistive switching behavior of such a device has been linked to the memristor circuit element theory originally predicted in 1971 by L. O. Chua. The discovery of the memristive behavior in the nanoscale switch has generated significant interest, and there are substantial on-going research efforts to further develop such nanoscale switches and to implement them in various applications. One of the many important potential applications is to use such a switching device as a memory unit to store digital data.
The devices recently reported using titanium oxide (and other metal oxides) typically have involved two oxide phases (TiO2 and an oxygen-deficient phase, TiO2-x). The two oxide phases (or one oxide phase, TiO2, bounded by two oxide phases, TiO2-x) are contacted by metal electrodes, typically, Pt, Ru, W, or other suitable metals.
One major issue in developing memristive nanoscale switching devices is that devices based on metal oxides as the switching material always required an “electroforming” process before they can be used for normal switching cycles. The electroforming process involves the one-time application of a relative high voltage or current to produce a significant change of the electronic conductivity of the device. Only after the forming can the device be operated as a tunable resistance switch that can go through repeated ON/OFF cycles. The electro-forming process is not a very well controlled process and is potentially destructive, as the conductivity change is very sudden, and the voltage at which the change occurs varies greatly from device to device. A greater problem, however, is that the formed metal oxide switching devices show a wide variance of operational properties, such as the voltage required for switching the device ON or OFF or to a selected resistance value, and the variations appear to depend on the details of electroforming. The wide variance in operational properties makes it difficult to use the switching devices in applications such as computing circuits. The lack of device repeatability can be a major barrier for the technological implementation of the nanoscale switching devices.